The present invention relates to a plasma burner equipped with two concentrically arranged electrodes and a nozzle surrounding the electrodes. The first one of the electrodes, known as the auxiliary electrode, is tapered at its front end. The second electrode is equipped with a central channel composed of a first cylindrical section, a conical taper and a second cylindrical section, narrower than the first, possibly followed by a wider section. The auxiliary electrode extends into the center of this central channel. The second electrode and the auxiliary electrode form an annular channel. The present invention also relates to a method for operating such a plasma burner.
In prior art devices, an electric arc, known as the auxiliary, or pilot, arc, is generated and maintained with the aid of a direct current source connected between the auxiliary electrode and the nozzle-shaped second electrode. Moreover, a gas jet is brought into contact with the auxiliary arc and is conducted through the nozzle electrode so as to drive the plasma formed by the auxiliary arc to a third electrode. The direct current source here serves not only to maintain the auxiliary arc between the auxiliary and second electrodes, but primarily also to heat the plasma generated by the electric arc and driven to the third electrode.
The current source between the second and third electrodes may be either a direct current source or, as disclosed for example in German Patent No. 1,440,594, an alternating current source. If an alternating current source is employed, the direct current arc burning between the auxiliary electrode and the second electrode serves to generate a continuous stream of ionized plasma and to bring this stream into the range of the primary, or main, arc which is maintained by alternating current. This is intended primarily to permit refiring of the primary arc after each zero passage of the alternating current and to additionally prevent thermal overloads on the nozzle electrode due to the alternating current spot burn at this electrode.
In addition to maintaining the alternating current arc and preventing thermal overloads on the alternating current electrode of a plasma burner, an auxiliary arc is often also required to start the plasma burner. This is done in that the plasma flame produced by the auxiliary arc and leaving the burner mouth forms a channel of ionized gas between the burner electrode and a counterelectrode or the material to be heated or melted, respectively, in which the primary arc--be it direct or alternating circuit--can begin to flow as soon as the primary arc voltage is applied between the burner electrode and the counterelectrode. For this purpose, the auxiliary arc may be generated, for example, between the burner electrode and the burner nozzle surrounding the burner electrode and forming the mouth for the burner material or between two auxiliary electrodes or also between the auxiliary electrode and the nozzle electrode of the above-described arrangement. Prerequisite for the initial firing of the primary arc is that the plasma firing flame extends to the counterelectrode or to the material to be melted and thus provides an uninterrupted electrically conductive path between the burner electrode and the counterelectrode. Consequently, the shorter the firing flame, the closer the plasma burner must be moved to the counterelectrode.
However, for many uses it is desirable or even necessary from an engineering point of view to fire the plasma burner from the greatest possible distance from the counterelectrode or from the material to be heated or melted, respectively, particularly if the material is bulky, such as scrap, for example, and the material does not present an essentially planar, but rather a craggy, surface. Since plasma burners must not come into contact with electrically conductive material as that would destroy them, it is of extreme importance in practical operation to be able to start the plasma burners at a safe distance from the surface of the material to be melted, i.e. with a correspondingly long firing flame.
If an electric arc arrangement as disclosed in German Patent No. 1,440,594 is employed, firing flame lengths of no more than 6 to 8 cm can be realized even if the primary arc current intensity and the plasma gas throughput are optimized. The device disclosed in German Offenlegungsschrift [Laid-open Application] 2,900,330 also does not result in sufficiently long firing flames.